This invention relates generally to spray cast metal matrix composites. More particularly, a copper or copper alloy matrix is interdispersed with a refractory metal or nonmetallic second phase during spray casting. Adhesion between the matrix and the second phase is enhanced by the addition of a reactive metal.
Metal matrix composites, having a metallic matrix and a second phase uniformly dispersed throughout the matrix can be tailored with unique properties. Some examples of these properties include second phase carbon or ceramic fibers to increase the stiffness and strength while reducing weight and the addition of second phase refractory metals to influence the coefficient of thermal expansion or softening resistance.
Differences in density, melting temperature and reactivity between the metallic matrix and the second phase make it difficult to form certain composites by conventional casting means. One casting method which has proven successful is spray casting. Spray casting is a method to manufacture metallic articles directly to a desired shape. The basic spray casting process comprises:
1. Atomizing a fine stream of molten metal.
2. Rapidly cooling the atomized droplets in flight so that the droplets are either at or near the solidification temperature.
3. Depositing the droplets on a collector. The collector is sometimes chilled to promote rapid solidification upon impact. Further, the collector moves in a predetermined pattern to generate a metal preform having a desired shape.
4. Optionally, working or directly machining the preform to generate the final shape and/or properties required.
The spray casting process is generally known as the Osprey Process and is more fully disclosed in U.S. Pat. Nos. RE 31,767 and 4,804,034, as well as United Kingdom Patent No. 2,172,900A, all assigned to Osprey Metals Ltd., of Neath, Wales.
The manufacture of composites by spray casting is disclosed in International Patent Application PCT/GB 88/01106 by Osprey Metals, Ltd. A metallic matrix is atomized by impinging a liquid metal stream with a high pressure gas. Second phase particles are added either to the liquid stream or to the post atomization divergent cone of metallic droplets. One example disclosed is atomizing titanium and adding 10 micron particles of silicon carbide. The titanium reacts with the silicon carbide precipitating titanium carbide dispersoids in a titanium matrix.
European Patent Application 88 305 050.2 by Alcan International, Ltd., discloses the manufacture of a metal matrix composite. The matrix is an aluminum/lithium alloy and the second phase is a particulate with an aspect ratio of less than 5:1. Disclosed second phases are silicon carbide, alumina and boron carbide. The metallic matrix is atomized and the second phase injected either into the melt or the droplets following atomization.
U.S. Pat. No. 5,120,612 by Ashok, discloses spray cast composite metals. The alloy matrix is a copper based alloy and the second phase is a nonmetallic refractory such as silicon carbide, alumina, titanium nitride, titanium oxide, titanium carbide or zirconium boride. The second phase is introduced as a solid into the atomized droplets. A reactive element such as zirconium, chromium or titanium is added to the metallic melt to improve the bond between the nonmetallic particles and the copper alloy matrix. U.S. Pat. No. 5,120,612 is incorporated by reference herein in its entirety.
While it is known to manufacture metal matrix composites by spray casting, several problems exist. The reactive elements may react with the walls of the furnace used to cast the melt, form excessive dross, or oxidize. Further, spray casting has proven generally unsuccessful for incorporating small and/or low density particles due to static electric clumping of the particles and low inertial momentum. Attempts to insert graphite particles into a copper alloy matrix by conventional spray casting means has been limited to about 1% by volume due to the aforementioned problems.
It is therefore Applicants' objective to provide a method for spray casting a metal matrix composite which does not have the problems of the prior art.